In the preparation of steel, iron, aluminum and galvanized surfaces for painting, the metal surfaces are frequently subjected to a pretreatment process referred to as phosphatizing. In this process, the metal surface is subjected to an alkaline cleaning, if necessary. It is subsequently rinsed, coated or immersed in a solution f a phosphatizing agent. Basically the phosphatizing agent is phosphate ion which may or may not contain additional metal salts. This dissolves a portion of the metal surface and forms phosphate salts on the surface. The phosphate salts act to prevent rust formation and improve adhesion of paint.
This does leave some voids in the phosphate coating which in the past have been eliminated or coated using a chromic acid final rinse. This eliminates the voids.
Unfortunately chromium presents an environmental hazard. Dispensing of the waste chromium causes significant problems and expenses. Further, chromium is dangerous to the worker.
There are many treatments for phosphatized metal surfaces. For example Guhde U.S. Pat. No. 3,877,998 and Kulick U.S. Pat. No. 4,039,353 disclose the use of a melamine formaldehyde composition. Linert U.S. Pat. No. 4,376,000 discloses another polymeric post-treatment and Schapira et al U.S. Pat. No. 4,497,666 discloses the use of a trivalent titanium compound. In a similar vein, Claffey U.S. Pat. No. 4,650,526 discloses treating a phosphated metal surface with an aqueous mixture of an aluminum zirconium complex comprising the reaction product of a chelated aluminum moiety, an organo functional ligand and a zirconium oxy halide. Zurilla in an article entitled "Phosphate Final Rinse Options" presented at "Pretreat 90" discloses a variety of different heavy metal, organic polymer, organic monomer, and inorganic post-treatments of phosphated metals.
Two important factors in selecting such treatments are cost and versatility. Many of these complexes are too expensive to effectively replace chromium. The various metallic complexes such as zirconium and the like tend to be relatively expensive. Versatility is also a significant function. One purpose of these treatments is to improve adhesion of paint to the treated surface and prevent the painted surface from rusting. Many polymeric coatings may function for treatment of surfaces which are to be coated with certain paints but not provide a good surface for other paints.
Environmental concerns are always significant. Although zirconium is not as objectionable as chromium, it is still a transition metal and it is preferable to avoid the use of transition metals.